As described in a patent document 1, a piezoelectric ultrasonic sensor fabricated by MEMS (Micro Electromechanical system) technique is known.
In the patent document 1, a piezoelectric vibrator is constructed with a ferroelectric interposed between a pair of electrodes arranged to face each other in a thickness direction of a substrate. The piezoelectric vibrator is formed on a thin portion of a surface of a semiconductor substrate to form an ultrasonic sensor element.
Further, in the patent document 1, a resonant frequency of the ultrasonic sensor is adjusted to a predetermined resonant frequency by applying a predetermined bias voltage between the pair of electrodes of the piezoelectric vibrator.
By the way, when a target object is moving relative to a device on which an ultrasonic sensor is mounted, a frequency of a reflected wave (a received ultrasonic wave) may be displaced from a transmission frequency due to Doppler effect of sound. In this case, the amount and direction of the displacement of the frequency are not uniform due to influences of the relative speed and direction.
Therefore, if an ultrasonic sensor is used for both transmission and reception, a frequency of a reflected wave is displaced from a transmission frequency even when a resonant frequency of the ultrasonic sensor is adjusted to a predetermined resonant frequency by applying a predetermined bias voltage between electrodes as disclosed in the patent document 1. As a result, a relatively moving target object cannot be accurately detected.